The piloting of a vehicle, such as an aircraft (or other advanced vehicles, i.e. a tank), is recognized as an extremely complex task because of the fact that the vehicle is free to move while exercising six degrees of freedom, in the case of an aircraft, including three degrees of translational freedom and three degrees of rotational freedom. In flying an aircraft, a pilot must reconstruct all six of these variables into a single trajectory and relate them to the three-dimensional world seen outside the windscreen. Some idea of the complexity of this task may be achieved by comparing it to the driving of an automobile, which is free to move with only two orthogonal degrees of freedom, thus resulting in only one angular degree of freedom which is, in any case, tied to the orthogonal movement, as long as no skidding is experienced. Display of vehicle instrument data in a head-up mode (head-up display or HUD) against the view through the windscreen may be achieved by placing a dichroic reflector in front of the windscreen and mounting a cathode ray tube with appropriate optical elements in the instrument panel of the vehicle to generate an image which may be reflected by the dichroic beamsplitter to be viewed by the pilot. It is noted that dichroic reflectors reflect only a selected wavelength while inhibiting that wavelength from being transmitted. After subtracting that wavelength from the outside world illumination, the contrast of the projected information is thus enhanced. For such purposes only employs the normal type of beamsplitter, known as a neutral density beamsplitter. Likewise, because of the properties of the dichroic reflector, the pilot may simultaneously see through the dichroic reflector and thus have a relatively unobstructed view through the windscreen. This system allows the addition of other functions, for example enhanced images can also be fed to the cathode ray tube, thus allowing the pilot to "see" during poor visibility conditions. Likewise, data with respect to the orientation of guns and the flight path of an aircraft could also be displayed by the system. Moreover, by appropriate selection of optical elements between the cathode ray tube and the dichroic reflector, the images projected on the dichroic reflector can be made to appear to be projected to optical infinity. Thus, the pilot can observe the instrumentation and other information while still focusing on the scene outside his windscreen.
In spite of the many obvious advantages of this system, it also has a number of problems which preclude its widespread employment in, for example, the aviation industry. Firstly, few existing aircraft have sufficient space available on an already crowded instrument panel to allow for the retro-fitting of this device. Secondly, even in the case of employment in newly designed aircraft, the demands for instrument panel space are such that the space is difficult or sometimes impossible to provide. Finally, there is also some objection to placing a dichroic reflector in front of the windscreen of an aircraft.
In an attempt to provide a head-up display system which does not have the disadvantages of the instrument panel dichroic reflector system described above, a system in which a television cathode ray tube is mounted on an aircraft pilot's helmet for viewing by the pilot has been developed. A display of the pilot's line-of-sight is also synthesized in this system, in order that slewable aircraft guns may be directed to fire along the pilot's line-of-sight onto a desired target. The pilot's helmet is necessarily provided with a device in which changes in a magnetic field created in the cockpit by a special apparatus are detected. The nature of the disturbance of the magnetic field tells an on-board computer the orientation of the pilot's helmet and, thus, his line-of-sight. This allows the slaving of guns or other systems on board the aircraft to movements of the pilot's head.
While this magnetic system solves the problems discussed above in connection with the instrument panel system, it does suffer from a number of its own significant problems. First, the pilot is forced to carry a great deal of heavy equipment on his head. This has prevented widespread adoption of the system inasmuch as most pilots find this unacceptable. In addition, in view of the fact that the helmet must be hard wired to a video generator inside the aircraft, this apparatus interferes with pilot ejection, should an emergency situation arise. Finally, the apparatus is very expensive, and because of its sophistication the cost of programming and aligning it is commensurate with the hardware cost. This is especially significant in view of the fact that alignment should be periodically checked and adjusted.